Princeton University Star Formation/ISM Rendezvous (SFIR)

ALMA observations have somewhat surprisingly revealed the presence of large amounts of dust in the first generations of galaxies in the Universe. Unfortunately, the dust temperature Td at redshift z>5 remains mostly unconstrained due to the few available FIR continuum data. This introduces large uncertainties in several properties of high-z galaxies, namely their dust masses (and thus dust production mechanisms), infrared luminosities (LIR), and obscured fraction of Star Formation Rates (SFR).

We have developed a new analytical method that allows us to constrain Td using a single continuum data point at 158 microns by combining it with the overlying CII emission. With our method, one can analyse uniquely the large number of CII and continuum detections at high-z provided by recent ALMA large programs. ALPINE and REBELS sources analysis allows us to extend for the first time the reported Td-redshift relation into the Epoch of Reionization (EoR). We find that Td does increase at higher redshift, but more mildly than previously suggested based on stacked SEDs fitting at z<4. We produce a new physical model that motivates the increasing Td(z) trend with the decrease of gas depletion time, tdep=Mg/SFR, induced by the higher cosmological accretion rate of galaxies at early times. The model also explains the observed Td scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. Moreover, Td is higher in metal-poor systems due to their smaller dust content, which for fixed LIR results in warmer temperature

A higher Td has testable implications: (a) it reduces the tension between local and high-z IRX-β relation, (b) it alleviates the problem of the uncomfortably large dust masses deduced from observations of some EoR galaxies, (c) it results in a larger obscured fraction of the SFR. This is particularly interesting given the recently proposed flattening of the cosmic Star Formation Rate Density (SFRD) at z>4, as indicated by observations at FIR and radio wavelengths of dusty UV-obscured systems.